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Tropical Storm Sergio Discussion Number   5
NWS National Hurricane Center Miami FL       EP212018
1000 AM CDT Sun Sep 30 2018

Recent microwave data indicate that the inner core of Sergio
continues to become better defined, with an eye-like feature
evident. However, this eye feature is not yet apparent in
geostationary satellite imagery. Deep convection with cloud top
temperatures of -70 C are concentrated around the storm's center as
well as along a prominent banding feature on the south side. The
average of available satellite intensity estimates support an
intensity of 50 kt, which will be the initial intensity for this
advisory.

The initial motion estimate is 270/11 kt. There continues to be no
significant changes in the forecast track philosophy from the
previous few forecasts. For the next couple of days, Sergio
should move generally westward or just south of due west
around the southern periphery of a large deep-layer subtropical
ridge that extends from Mexico westward across the eastern Pacific.
By 48 hours, a mid- to upper-level trough will move southeastward
along and offshore the western U.S., which will erode the ridge from
the west. This will allow the cyclone to turn west-northwestward
then northwestward from 48- to 96-hours with a gradual decrease in
forward speed. Thereafter, the trough should shift eastward and low-
to mid-level ridging should build to the northwest of the cyclone.
This pattern change should induce a more westward turn toward
the end of the forecast period.

Sergio will remain in an environment of low-shear, high-moisture,
and over warm waters for the next 24 hours or so, which should
result in rapid intensification. This is in agreement with rapid
intensification indices from the SHIPS model of 40 to 45 percent
over the next 24 hours. By 36 hours, northeasterly shear is expected
to develop over the storm environment and increase to 25 kt by 48
hours. This should at least limit the rate of intensification if not
weaken the cyclone slightly from 48- to 72-hours. After that time,
global models indicate that the shear will lessen to under 10 kt by
96 hours, but by that time the system will be moving over waters of
26-27 C very near a gradient of cooler SSTs just to the north. Based
on this, the intensity remains steady from 72- to 96-hours and then
shows slow weakening by the end of the period. The intensity
forecast is a little higher that the previous advisory, and is close
to the dynamical model consensus. Based on this intensity forecast,
Sergio is forecast to become a major hurricane by Tuesday.

FORECAST POSITIONS AND MAX WINDS

INIT  30/1500Z 12.1N 105.4W   50 KT  60 MPH
 12H  01/0000Z 12.1N 107.0W   65 KT  75 MPH
 24H  01/1200Z 11.8N 109.2W   80 KT  90 MPH
 36H  02/0000Z 11.7N 111.4W   95 KT 110 MPH
 48H  02/1200Z 12.0N 113.5W  105 KT 120 MPH
 72H  03/1200Z 13.6N 116.6W  100 KT 115 MPH
 96H  04/1200Z 16.0N 119.0W  100 KT 115 MPH
120H  05/1200Z 17.5N 121.5W   90 KT 105 MPH

$$
Forecaster Latto/Cangialosi

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